Abstract: A gas cooled nuclear reactor is permanently deactivated on occurrence of an emergency by pyrolytic deposition of boron or refractory or stable compounds of boron in the fluid channels of the fuel elements of the reactor. The boron is enriched in boron 10. The deposition takes place in so short a time interval as to preclude a major catastrophe by reason of penetration of water into the reactor. Carbon and/or nitrogen-containing boron compounds are injected directly into the reactor or compounds generated by reaction in the working fluid of the reactor between diborane and other boron hydrides with unsaturated compounds, such as acetylene and ammonia flow through the reactor. The compounds are carried by the working fluid through the core and are pyrolized in the heat of the core to produce more stable boron, or boron-carbon and boron-nitrogen compounds or metal borides which adhere to the walls of the channels in the fuel elements, deactivating the core.

Abstract: A fuel arrangement for a high temperature gas cooled reactor including fuel assemblies with separate directly cooled fissile and fertile fuel elements removably inserted in an elongated moderator block also having a passageway for control elements.

Abstract: An improved nuclear fuel particle comprises (a) a generally spherical low density kernel of a fissile element having a void volume of at least 40 percent of the physical volume of the kernel in order to accommodate fission gases generated during the use of the fuel particle, (b) the kernel is coated over its entire surface with an extremely thin carbon buffer layer to provide for fission recoil, and (c) thin pyrolytic carbon, (d) silicon carbide and (e) outer pyrolytic carbon layers are superimposed thereon to hermetically seal the kernel to prevent escape of generated fission gases, the pyrolytic carbon and silicon carbide layers being of sufficient strength to resist rupture of the pressure of the contained fission gases even at high reactor temperatures of the order of 1000.degree. C-1300.degree. C and a burn up of 50 percent or more FIMA. Such particles can be embedded in graphitic bodies to provide fuel elements having a greatly increased fuel loading per unit volume of particle and fuel element.